Most residential construction includes an attic space between the ceiling and the roof deck. The structure that supports the roof and provides the ceiling plane is often constructed with pre-assembled wood trusses. The structure can also be built on-site using traditional ceiling joists and roof rafters. Properly insulating the attic is essential to reducing home energy consumption (“building load”). Thermally isolating the attic from the rest of the house also increases the comfort of the living space below in both winter and summer.
Attic ventilation serves two purposes: prevention of moisture condensation in the winter and attic cooling in the summer. Ventilation during the heating season removes moisture-laden air from the attic before it condenses. Summer-time venting allows cooler air to flush heat out of the attic space. Typically, a good ventilation system has 50 percent of the required ventilation area high on the roof and 50 percent in the eave area. With properly spaced vents, the attic will have good circulation. Batt insulation is often a good bet for long-term thermal performance. Loose-fill insulation may also be used, including cellulosic and glass fiber loose-fill insulation. Loose-fill insulation should be installed at the same thickness throughout the attic. Voids and low spots should be eliminated.
Despite the many attempts to properly insulate attics, the attic space of most buildings is perceived as a source of a nuisance. In winter, moisture condensation on the attic ceiling encourages mildew growth. In summer, the heat build-up in the attic space increases the cooling load. Generally during the night, the high attic air relative humidity is caused by an air exchange with the outdoor environment. The wood framing materials, generally having a lower air relative humidity at the surface, adsorb moisture. During the daytime the attic air relative humidity is reduced due to the heat gain caused by solar heat. There is a higher air or relative humidity at the surface of the wood framing materials which results in moisture being desorbed by wood attic framing materials. The attic space during the daytime will typically be elevated in relative humidity above the outdoor environment.
In modern residences, the challenge of achieving a continuous air infiltration barrier and thermal insulation barrier at the interior ceiling level is especially difficult. The air barrier, used to isolate the living space from the attic, is usually the taped drywall ceiling, while the thermal barrier is the insulation placed on top of the drywall. The ceiling may not be a single horizontal plane, but a series of horizontal planes, vertical planes (knee walls), and sloped planes, all intersecting to create the ceiling.
Some building codes across the United States currently require attic ventilation. Lstiburek, “Vented and Sealed Attics in Hot Climates” ASHRAE SYMPOSIUM ON ATTICS AND CATHEDRAL CEILINGS, TORONTO, (JUNE, 1997). In cold climates, the primary purpose of the attic ventilation is to maintain a cold roof temperature to avoid ice damage created by melting snow, and to vent moisture that moves from the conditioned space to the attic. Id., p. 3 Melted snow, in this case, is caused by heat loss from the conditioned space. When water from melted snow runs out over the unheated eave portion of the house, it freezes and expands, often driving its way back up the roof and between shingles. Id.
In hot climates, the primary purpose of attic ventilation is to expel solar heated hot air from the attic to lessen the building cooling load. Id., p. 4 Roof shingle temperatures will be higher during no-wind conditions leading to a higher heat load on the attic. Id. Therefore, the greatest need for attic ventilation is when there is a little wind pressure to force air in and out of the attic.
Building heating and cooling loads from roofs for all single-family detached buildings were estimated to be 446 trillion BTUs for heating and 128 trillion BTUs for cooling in the U.S. in 1999. Y. J. Huang et al. “Residential Heating and Cooling Loads Components Analysis,” LBNL-44636, Lawrence Berkeley National Laboratory, Berkley, Calif., (1999). Compared to the total net heating and cooling loads (through roof plus walls plus floors), the percent of roof loads are 15.1 percent (heating) and 15.8 percent (cooling), respectively. Therefore, roof load reduction can greatly reduce the total building loads. Although there are many ways to reduce roof loads, the most common way is to add more roof insulation. Due to limited attic space, adding more roof insulation may not always be a feasible way to reduce the total building loads in many instances.
Accordingly, there remains a desire to reduce total energy usage by alternative means.